1. Field of the Invention
The present invention relates to a power factor correction (PFC) device, and to power converter which incorporates a PFC device.
2. Description of Related Art
The requirement for loads connected to the AC mains to draw power with low harmonic distortion has existed for many years. In Europe, EN61000-3-2 is the relevant standard for single phase power supplies. EN61000-3-2 sets different limits, depending on the end equipment.
For industrial applications, EN61000-3-2 defines limits in terms of absolute levels of harmonic content. In practice, power supplies which operate at up to 100 W, with a power factor of around 0.6, comply with these absolute limits without a need for additional power factor correction. However, at higher power levels, a higher power factor, close to unity, is required.
For lighting and computer equipment, EN61000-3-2 defines more stringent harmonic current limits, such that a power factor close to unity is required at lower power levels than for industrial applications.
A common power factor correction technique, aimed at achieving compliance with EN61000-3-2, involves shaping the input current to be nearly sinusoidal. For lower loads, some level of cross-over distortion in the input current waveform can be tolerated whilst still meeting the legislative requirements. Thus, PFC circuits which create other input waveforms and achieve compliance with EN61000-3-2 are also known. However, square wave or quasi-square wave waveforms with steep edges are generally avoided, since these tend to have high harmonic content, and can also result in increased levels of audible noise.
Previous attempts to achieve compliance with harmonic content legislation have achieved success at the expense of reduced efficiency, due, for example, to increased switching losses.
U.S. Pat. No. 7,295,452 discloses a boundary-conduction-mode (BCM) PFC circuit which uses phase control. The circuit operates at a switching frequency which is approximately inversely proportional to the instantaneous input current. To avoid excessively high frequencies around the mains zero crossings, operation is started and stopped symmetrically around the peak of the mains, which results in a sinusoidal input current, with part of the waveform near the zero crossings missing (ie zero current), and steep rising and falling edges where the converter is started and stopped. These steep edges result in undesirable higher order harmonics.